1. Field of the Invention
The invention relates to a closing apparatus for closing bag-type packaging units by compression, which units have regions of differing thickness along the package opening to be closed, such as square bottom bags for instance, in particular made of coated or uncoated paper, paperboard, plastic, film, and/or woven material, which packaging units have at least one suitable sealing surface that is integrated in or applied to the material for permanent bonding to a surface that is adjacent to or opposite and pressed against the material, wherein the closing apparatus has a pressing device with at least one pressure device and at least one counter-pressure device between which the packaging units can be compressed, with the application of heat as required, in the region of their sealing surfaces, wherein the pressure device and counter-pressure device have zones of different pressure and/or different temperature along the package opening to be closed.
2. Description of the Background Art
In the field of the packaging industry, automated processing lines are employed to fill and subsequently seal various packaging units, wherein the packaging units are either supplied to the processing line as prefabricated empty containers, or the packaging material is supplied to the processing line as raw material (for instance, as sheets of paper or paperboard, or as film wound on a roll) and the packaging units are fabricated immediately prior to filling, for instance by welding the rolled-up packaging material in the form of a bag or by folding and adhesive bonding the sheets.
Bulk materials place special demands on the packaging units and the packaging equipment, especially when they have to be packed in the form of a fine powder, as is frequently the case for, e.g., construction materials or other granular or powdered materials, for instance in the pet goods industry. Moreover, these materials have to be packed in very large quantities, with sacks made of (coated or uncoated) paper or paperboard, or (sometimes fiber-reinforced) plastic usually being used. Because of the large fill quantities and the resultant high weight, these sacks generally are subjected to high stresses, yet should not tear or break open too easily, even when handled roughly on construction sites.
Especially high strength can be achieved with square bottom bags; not only are these bags very sturdy, but they also can be stacked and palletized especially well in the filled state because of their squared-off shape. Furthermore, the folded and still-unfilled square bottom bags stack well, so that they generally are supplied in prefabricated form, and delivered to the packaging line in stacks, where they are then separated and unfolded, in which process they usually are removed from the stack by a suction device, opened, and placed on a conveyor, and transported by the conveyor to the individual processing stations. Generally speaking, these processing stations include: a filling assembly for placing the powdered material in the sacks; a vibrator assembly for preventing air pockets in the powder material; a folding device, which presses together the opening of the bags from both sides, in which process it is necessary to ensure that the side panels of the square bottom bags are correctly folded inward; a trimming device for trimming the upper edge of the bag; a sealing unit, which tightly seals the bag opening, for example by hot-melt adhesive bonding, folding, sewing, pressing, and/or adhesive bonding; and a palletizing device, which stacks the filled and completed sacks on pallets.
With many of the packaging and sealing systems currently in use, only a relatively low cycle rate can be achieved for the present area of application, roughly in the range of 10 to 20 packed bags per minute and packaging line, with developments in recent years having accelerated these processing speeds only to a minor and unsatisfactory degree. Significant potential for improvement can be anticipated, especially with the sealing units that typically are used.
In the case of square bottom bags, for instance, sealing surfaces are provided on the inside of the material layer of the bag in the region of the opening; usually these sealing surfaces have one or more layer(s) of a hot-melt adhesive. To seal them, the bags are pressed together at their sealing zone and heated, and are bonded together in this way. To this end, first the side panels of the previously filled bags are folded inward by a device, and then the top part of the bag is pressed together, for instance by converging guide rails. For the actual sealing process, the sealing region of the bag is then placed between a pressure plate and a counter-pressure plate, which are subsequently brought together in order to compress the bag opening between them. Depending on the material used for the sealing surface or for the bag, either the pressure alone may suffice to seal the bag opening by means of pressure welding, or the sealing zone is additionally heated by heating elements provided at the plates in order to achieve self-adhesion or to soften a hot-melt adhesive applied in the sealing region. In order to weld the sealing surfaces on the inside of the packaging together tightly, the pressure must be maintained for a sufficient length of time. In this context, the compression time is related to the material to be sealed, the pressure applied to the material, and the temperature applied. The minimum required compression time is critical to the cycle times that can be achieved with this method, and at the present time is typically in a range from 3 to 7 seconds, and is empirically determined for each production run with the predefined pressing force. After pressing, the plates are opened and the closed and sealed packaging unit is transported to the next processing station by a conveyor belt.
In most cases, the process parameters necessary for tight sealing of the bags, thus in particular the temperature, pressure, and pressing time, can only be determined empirically, as already noted. The primary reason for this is that the manufacturers oftentimes do not disclose specific material characteristic values for the sealing materials that they supply, such as hot-melt adhesives, and these characteristic values can sometimes differ from one manufacturing lot to the next. The packaging materials also vary with regard to their physical characteristics. As a result, it can be necessary to adjust the parameters of the sealing unit to new conditions, even though the same product from the same manufacturer (for example, ready-made square bottom bags) is still being processed. A systematic approach to making these adjustments is made more difficult by the fact that no sensors for measuring the actual pressure and the actual temperature in the sealing zone are generally provided as yet in the sealing equipment currently offered on the market.
Another problem arises in packaging units that have different material thicknesses across the sealing region. This is the case in square bottom bags, in particular, which have a four-fold material thickness on the edge due to the inwardly folded side panels, but have only a two-fold material thickness between the folded side panels. This can be the case in other packaging units as well, for instance because of handle reinforcements or carrying handles embedded in the sealing zone.
To remedy the aforementioned problem, DE 3136936 A1 discloses a device for applying transverse welded seams to thermoplastic tubular films having gussets folded inward at the longitudinal edges, wherein this welding device has sealing jaws that are subdivided in such a manner that it is possible to maintain a larger spacing between the pressing and welding jaws in the region of the inwardly folded gussets than in the center region between the inwardly folded gussets.